enhance(postman): comprehensive steering guidance for Postman MCP workflows

[christosgkoros]

Summary

• Replaces the minimal 3-section steering file with comprehensive, always-included guidance authored by the Postman team
• Switches authentication from API key to OAuth — no manual key setup needed; Kiro handles the sign-in flow automatically
• Covers schema validation requirements, variable best practices, resource discovery/deduplication, and 4 named workflow patterns (create, test, generate from OpenAPI, sync)
• Adds rich test script patterns, .postman.json config format, CI/CD integration guidance, and troubleshooting reference

What changed

mcp.json

• Removed Authorization: Bearer ${POSTMAN_API_KEY} header
• Server now relies on OAuth (MCP spec–compliant, supported natively by Kiro)

POWER.md

• Removed Step 1 onboarding (API key setup instructions)
• Updated authentication description: "OAuth — Kiro will open a browser sign-in on first use. No API key required."
• Updated server metadata: OAuth instead of Bearer token
• Corrected Full mode URL: /mcp (was incorrectly /full)
• Replaced "Invalid API key" troubleshooting with OAuth re-auth guidance
• Updated config example: no headers block

steering/steering.md

• Replaced minimal 3-section file (~25 lines) with comprehensive Postman MCP workflow guide
• inclusion: always — loaded into every agent context when the power is active

Background

OAuth support (https://mcp.postman.com) is now the recommended authentication method per the Postman MCP Server README.